Thirlmere is a glacial valley, with good water catchment fells either side and streams running down into the valley bottom. These features make Thirlmere one of the purest water sources in the country. Today this same aqueduct takes water to many other places on route to Manchester as well as being supplemented by water from Haweswater and many other smaller sources. 220 million litres of water per day flow through the Water Treatment Works sited 3 miles south of the reservoir.